Most societies throughout the world promote monogamy as the ideal relationship state. As a result, a large majority of people have come to believe that any form of sexual activity with someone other than one's current romantic partner is unacceptable and morally wrong [1]. For instance, United States public opinion polls indicate that 88% of adults think having an affair is immoral. Despite how widely this belief is held, people do not seem to practice what they preach, given that we are confronted with media headlines almost every day about the latest celebrity or political figure caught having an affair. So just how common is cheating?

This question is difficult to answer because it depends how you define “cheating,” the type of relationship you’re talking about (e.g., dating vs. married), and a whole host of other factors. For example, a recent review of 31 different studies of infidelity conducted between 1982 and 2009 found that anywhere from 1.2% to 85.5% of respondents reported cheating on a partner before [2]! How can these numbers be so discrepant? If we look closer at the data, we find that cheating tends to be more frequent among men than women and also among couples who are dating compared to those who are married. We also find that emotional infidelity (e.g., developing feelings for or flirting with someone other than your partner) tends to be more common than physical infidelity (e.g., having sex with someone other than your partner). Lastly, the prevalence of cheating is higher when you ask people if they’ve ever done it before compared to whether they have done it in their current relationship.

Based upon the questions I have been asked over the years, people seem most curious about the percentage of married people who have had sex outside of their current relationship. Research reliably puts that number at around one in four or one in five (at least in the United States--the numbers vary substantially across countries) [2]. The other thing people often want to know is how many college students have ever been sexually unfaithful to a partner. Research pretty consistently puts this number around one in two or one in three [2]. However, keep in mind that, if anything, these are probably underestimates because not everyone who cheats is willing to admit to it, even on an anonymous survey.

In short, cheating is a common activity, but the prevalence of it varies widely depending upon who you ask and how you ask the question.

For more information on cheating, including some of the reasons people do it and the outcomes associated with it, see here.